[texdoc] configuration items doc

[Karl Berry]

Hi Manuel,

I suggest alphabetizing the list of names in the "Configuration items"
section; after all, it's supposed to be the reference.

I also suggest making the first section, Structure of configuration
files, be text at the beginning of the section instead of being a
subsubsection.  That way all the subsubsections will be individual
config items, which somehow seems more parallel, or something.

And, the copyright years in the last section ("Licence") need to be
updated.

Enough for this time :),
karl